Aromatic compounds have a multitude of uses, both as end products and as reactants for downstream processes. Methods of preparing aromatic compounds from a hydrocarbon feed are generally known in the art and include upgrading the hydrocarbon feed followed by reforming and aromatics separation. Typical upgrading techniques include hydrotreating to remove contaminants such as sulfur, nitrogen, and oxygen. After upgrading, the hydrocarbon feed is reformed in the presence of a catalyst to convert paraffins and naphthenes to a reformate that includes aromatic compounds such as xylenes, benzene, and toluene. A series of separation techniques are employed to separate the various aromatic compounds from the reformate, and numerous product streams having varying degrees of purity may be isolated for each aromatic compound in the reformate.
Toluene is a common aromatic compound that has many uses not only as an end product, but also as a process stream during production of other aromatic compounds. Toluene is generally separated from a reformate in a toluene column that is downstream of a benzene column. The toluene column fractionates benzene-depleted reformate into a toluene-containing stream and a xylene-containing stream. The toluene-containing stream can be blended with C9 or greater aromatic compounds for conversion into xylenes and benzene through disproportionation and transalkylation. The xylenes and benzene produced through disproportionation and transalkylation can be separated along with the reformate through conventional separation techniques.
Adsorption/desorption is a common separation technique that is employed for separation of xylene isomers, such as para-xylene, meta-xylene, and ortho-xylene. During adsorption/desorption, select xylene isomers, such as para-xylene or meta-xylene, are adsorbed from a xylene-containing stream that is generally depleted of benzene and toluene. Specific xylene isomers can be selectively adsorbed by selecting appropriate adsorbing material. A desorbent, which can be readily separated from adsorbed compounds through fractionation, is generally employed to remove adsorbed isomers from the adsorbent material. Raffinate from adsorption/desorption generally also includes the desorbent, and the desorbent is generally separated from the raffinate through fractionation to recover the desorbent for further use.
Due to different compositional makeup of the raffinate and the reformate, the raffinate and the reformate are generally fractionated through separate fractionation techniques to separate individual compounds therefrom. Separate fractionation is conducted even when a desorbent such as toluene is used and is present in the raffinate. For example, whereas the reformate can be fractionated in the toluene fractionation unit to produce a bottoms stream that includes a range of xylene isomers, the raffinate from adsorption/desorption is generally depleted of para- and/or meta-xylenes such that mixing of the raffinate with the reformate would dilute the content of para- and/or meta-xylenes in the bottoms stream from the toluene fractionation unit. Instead, the raffinate is separately fractionated from the reformate to produce a bottoms stream that contains any xylenes that remain after adsorption/desorption, such as ortho-xylene, and the ortho-xylene may be isomerized to produce para-xylene or meta-xylene. Separate fractionation columns and associated units such as receiver vessels and overhead pumps are thus required for fractionating the reformate separate from the raffinate.
Accordingly, it is desirable to provide methods and apparatuses for separating toluene from multiple hydrocarbon streams, such as a hydrocarbon stream including reformate and a hydrocarbon stream including raffinate from para-xylene and/or meta-xylene adsorption/desorption processes, that enable duplication of fractionation equipment to be minimized. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.